When doing builds like this you basically have two options, one is to build a bike from scratch with the major parts, i.e. frame, engine, gearbox, etc, etc all new & then get it SVA, (Single Vehicle Approved) tested. This is basically an indepth MOT, but also checks things like lights being E-marked, at the correct heights above the ground, distance from the vehicle centre, ball-ends on your handlebar levers & a whole host of other requirements. For that you get a brand new registration, (if it passes !!) & do not need an MOT for three years. Basically exactly the same & buying a brand new bike from a dealer.
The other option is to buy a bike & modify it, or buy a bike frame with a V5 & modify that. I chose that route
(Just added) Sorry Gimlet, I forgot to include info about the engine, it's a brand new S&S P-series Panhead copy. Basically a modern Panhead built to look like the old Panheads.
Out of the various versions of Harley engine it's the one I like, aesthetically, the best followed by the Knucklehead, but Knuckleheads command higher prices whether they are modern versions or originals. Both these versions have the "bulge" on the front right-hand side of the crankcases where the generator would originally have been mounted.
Having said that the Shovelhead seems to be the most popular & the cheapest no matter what era it comes from, maybe because it's cheaper. These engines don't tend to have the generator "bulge".